Large boron hydride compounds have become important feed stocks for boron doped P-type impurity regions in semiconductor manufacture. More particularly, high molecular weight boron hydride compounds, e.g., boron hydride compounds comprising at least a five (5) boron atom cluster, are preferred boron atom feed stocks for boron atom implantation.
An important aspect of modern semiconductor technology is the continuous development of smaller and faster devices. This process is called scaling. Scaling is driven by continuous advances in lithographic process methods, allowing the definition of smaller and smaller features in the semiconductor substrate which contains the integrated circuits. A generally accepted scaling theory has been developed to guide chip manufacturers in the appropriate resize of all aspects of the semiconductor device design at the same time, i.e., at each technology or scaling node. The greatest impact of scaling on ion implantation processes is the scaling of junction depths, which requires increasingly shallow junctions as the device dimensions are decreased. This requirement for increasingly shallow junctions as integrated circuit technology scales translates into the following requirement: ion implantation energies must be reduced with each scaling step. The extremely shallow junctions called for by modern, sub-0.13 micron devices are termed “Ultra-Shallow Junctions” or USJs.
Methods of manufacturing boron doped P-type junctions have been hampered by difficulty in the ion-implantation process using boron. The boron atom, being light (average atomic weight of 10.8), can penetrate more deeply into a silicon substrate and diffuse throughout the substrate lattice rapidly during annealing or other elevated temperature processes.
Boron clusters or cages, e.g., boranes have been investigated as a feed stock for delivering boron to a semiconductor substrate with reduced penetration. For example, as recited in commonly assigned International Patent Application PCT/US03/20197 filed Jun. 26, 2003, boron ions may be implanted into a substrate by ionizing boron hydride molecules of the formula BnHm (where 100>n>5 and m≦n+8) and an ion source for use in said implantation methods. Certain preferred compounds for use in the boron ion implantation methods included decaborane (B10H14) and octadecaborane (B18H22).
A typical molecular ion beam of B18H22 contains ions of a wide range of masses due to loss of a varying number of hydrogens from the molecular ion as well as the varying mass due to the two naturally occurring isotopes. Because mass selection is possible in an implanter device used in semiconductor manufacture, use of isotopically enriched boron in B18H22 can greatly reduce the spread of masses, thereby providing an increased beam current of the desired implantation species. Thus, B-11 and B-10 isotopically-enriched B18H22 is also of great interest.
B18H22 can be prepared by the oxidation of alkylammonium salts of the B10H10 dianion. Preparation of this dianion can be accomplished in high yield from decaborane (M. F. Hawthorne and A. R. Pitochelli J. Am. Chem. Soc. 81, 5519, 1959.). However decaborane is toxic, expensive and difficult to prepare by reported synthetic procedures (see, U.S. Pat. No. 4,115,521, issued to Dunks et al.). More particularly, the Dunks method of synthesis of decaborane employs costly solvents and reagents, time consuming reaction conditions, and often laborious work up procedures. Thus, the overall yield for the preparation of salts of the B10H10 dianion starting from sodium borohydride and proceeding through decaborane is typically below 30%.
Preparation of B-10 and B-11 enriched salts of the B10H10 dianion, and preparation of large boron hydrides from salts of the B10H10 dianion (such as B18H22), via enriched decaborane is a particularly expensive process in part because substantial quantities of preparation of B-10 or B-11 enriched from sodium borohydride is diverted to byproducts instead of incorporation into the enriched decaborane and enriched B10H102− dianion.
International patent application WO 03/044837, (Applied Materials, Inc, Santa Clara Calif.) recites methods of ion implantation in which an isotopically enriched boron compounds including 11B enriched compounds are ionized and then implanted into a substrate. The '837 publication recites the preparation of the iosotopically enriched boranes by the method recited in U.S. Pat. No. 6,086,837 (Cowan, et al.), which methods are reported to be the current industrial process for the preparation of boranes isotopically enriched in 10B or 11B.
Cowan (U.S. Pat. No. 6,086,837) recites a method of preparing B-10 enriched decaborane starting with B-10 enriched boric acid. The Cowan preparation of either B-10 or B-11 enriched boron hydrides begins with boric acid and involves a multitude of synthetic and purification steps. More particularly, the Cowan process for conversion of boric acid into an alkali metal borohydride involves numerous time consuming steps and results in a relatively low yield of valuable B-10 enriched borohydride which must then be subjected to further reactions to obtain final product.
Thus, the Cowan method starts with the preparation of B-10 methylborate from boric acid and methanol using an azeotropic distillation method. The methylborate is separated from remaining methanol by freeze recrystallization by means of three one step procedures to produce an 80% yield of trimethylborate. The trimethylborate is then added to a suspension of sodium hydride in mineral oil at 220° C.-250° C. and heated for 12 hrs. For safety, a metal reflux condenser is required. Isolation of the formed borohydride requires special attention. First, the excess sodium hydride is destroyed by pouring the mineral oil mixture into a mixture of ice and water, a rather exothermic process evolving gaseous hydrogen. Then the aqueous borohydride is separated from the mineral oil by decantation or use of separatory funnel. The aqueous borohydride must be purged of methanol by either heating to 60° C. and purged with a nitrogen stream or by removal under reduced pressure. The resulting aqueous solution is comprised of sodium hydroxide and the B-10 enriched borohydride. Carbon dioxide gas is bubbled through the solution converting the sodium hydroxide to sodium carbonate. The resulting slurry is then extracted with n-propylamine and the n-propylamine evaporated to yield final product. The solubilty of sodium borohydride in n-propylamine is limited and appreciable volumes of the volatile solvent are needed. Typical yields of 45-65% are obtained. A total of ten time consuming steps are required to prepare isotopically enriched sodium borohydride by the procedure recited in Cowan.
Several literature documents recite conflicting synthetic reports regarding the preparation of salts of the B10H102− anion from tetralkylammonium borohydride salts. The literature recites conducting the pyrolysis in a variety of reactors, in the presence or absence of a solvent, and under a variety of reaction conditions. See, for example, (1) W. E. Hill et al, “Boron Chemistry 4.” Pergamon Press, Oxford 1979, p 33; (2) Mongeot et al Bull. Soc. Chim. Fr. 385, 1986; and (3) U.S. Pat. Nos. 4,150,057 and 4,391,993, issued to Sayles. The published procedures do not provide the means for industrially significant production of the B10H102− anion, predictable and consistent conversion to product are not taught, and purification techniques are inadequate for the intended use.
Several reports have recited processes for the preparation of naturally abundant tetraalkylammonium borohydride compounds from sodium borohydride. However, the literature methods are not suitable for preparation of isotopically enriched ammonium borohydrides, in part because, a substantial amount of the borohydride is sacrificed during cation exchange. For example, Gibson and Shore separately recite contacting two equivalents of sodium borohydride with a mixture of tetraethylammonium hydroxide and sodium hydroxide in methanol to generate one equivalent of tetraethylammonium borohydride, which may be contaminated with sodium hydroxide (D. Gibson et al, J. Organornet. Chem, 218, 325, 1981; and S. Shore et al., Inorg. Synth. 17, 21, 1977). Due to the stoichometric loss of boron, these processes are not suitable for preparation of B-10 or B-11 enriched tetraalkylammonium borohydride salts.
Brändström et al recites methods of synthesis of tetralkylammonium borohydride compounds containing 12 or more carbon atoms from tetraalkylammonium hydrogen sulfate and a 10% excess of sodium borohydride (Brändström et al Tet. Lett. 31, 3173, 1972). Notwithstanding the quantitative conversion to the desired production solution, Applicants attempts to isolate the product tetralkylammonium borohydride from solution were plagued by unsatisfactory isolated yields and development of viscous “oils” that were difficult to crystallize and purify following the recited procedure.
It would be desirable to have a reproducible, atom-efficient, high-yielding process for preparing high-purity salts of the B10H102− dianion from borohydride precursors. More particularly, it would be desirable to have methods of preparing high purity natural abundant, B-10 enriched, or B-11 enriched salts of B10H102−, which methods have a reduced number of synthetic procedures.